Electroporation has become a valuable technique for the transfer of DNA into eukaryotic and prokaryotic cells. It is currently the most efficient means of transferring DNA, with efficiencies 20 times higher than obtained with maximally competent cells prepared with calcium chloride (CaCl₂). Chemical transformation using CaCl₂ is generally used for routine transformation when the DNA sample is not limiting. Electroporation is particularly useful in applications where high transformation efficiencies are specifically required. In essence, fast growing cells are centrifuged and washed in ice-cold water. They are placed in a cuvette with the DNA and a high-voltage pulse is applied. Cells are then plated out and selected for on agar.